This invention relates to an inkjet head that ejects ink droplets in a continuous and consistent way. It also relates to an inkjet printer using the inkjet head.
Inkjet printers are commonly used today to record image by ejecting ink droplets onto paper and other recording media.
The inkjet head used on those inkjet printers has a plurality of ink ejection nozzles provided adjacent to each other in one direction, with a discrete ink channel being connected to each of those ink ejection nozzles such that ink is kept supplied to them. In that area of the discrete ink channel which is in the neighborhood of each ink ejection nozzle, a means of ejecting ink droplets such as a heater or a diaphragm utilizing a piezoelectric device made of PZT is formed in association with each ink ejection nozzle. By this ink ejection means, ink is locally heated and the evolving bubble is expanded or, alternatively, mechanical vibration is applied to the ink, whereupon the latter is sufficiently pressurized to eject an ink droplet through the ink ejection nozzle.
For fast recording of image and other information by the above-described inkjet head, it is desired to increase the frequency at which ink droplets are ejected. However, in order to ensure that ink droplets are ejected consistently at high frequency, it is important that the volume of the ejected ink droplet be rapidly compensated by supplying the necessary amount of ink toward the ink ejection means (this step is called ink refill). In the currently available type of inkjet head, ink refill is accomplished by restoring the meniscus on the ink through the use of its small surface tension. This approach has the problem of requiring prolonged time to restore the meniscus because, for one thing, the restoring force of the meniscus is small and, for another, the inertial resistance of ink movement is large. In addition, any vibration of the meniscus will damp only slowly, so a statically determinate meniscus cannot be restored rapidly enough by ink refilling.
With a view to solving these problems, JP 8-132613 A discloses an inkjet recording apparatus in which a common fluid compartment is pressurized in synchronism with the timing of ink refill in order to reduce its capacity, whereby ink is pressurized toward a heater as an ink ejection means by a sufficient degree to realize rapid ink refill.
The problems are also addressed in JP 2001-205814 A which discloses an inkjet head comprising a substrate having a plurality of heating resistors arranged in a straight row and a top plate spaced from the substrate and which has a plurality of ink ejection orifices, wherein ink is circulated between the substrate and the top plate in such a way that it flows in a direction perpendicular to the arrangement of the ink ejection orifices.
However, the inkjet recording apparatus disclosed in JP 8-132613 A has the problem of increased cost since a piezoelectric device that pressurizes the common fluid compartment in synchronism with the timing of ink refill and a diaphragm that is activated by the piezoelectric device to change the capacity of the common fluid compartment must at least be constructed separately. In addition, a control circuit for controlling the piezoelectric device in synchronism with the timing of ink refill must be added and it is difficult to control the piezoelectric device.
The inkjet head disclosed in JP 2001-205814 A is believed to accomplish efficient ink refill by keeping constant ink flow. However, in order to circulate the ink, a circulating pump or other suitable means is required, leading to a higher cost of inkjet printer.